Board Game Arena

I have had issues where people have two accounts and log in on both on different computers and cheat?

That is certainly a problem if you suspect people of cheating, but your question asks about spectators. Are you talking about people not actually involved with the game? In that case, they can't see anything that the specific player can't. If you suspect someone is using multiple accounts, go to that player's profile page and report them to the admins.